Yeah, Duktape is great. It has a very active developer and evolves quickly as a result. He's also very accommodating of feature suggestions and pull requests, which allowed me to develop minisphere very quickly. I started development on the engine in January and it's already at v2.1.1...

Also thanks to minisphere, Sphere is a proper development platform now--the GDK includes not only the engine, but a full-featured single-step debugger for Sphere Studio as well as a scriptable SCons-inspired build tool called Cell (which can also create SPK packages). You should try it out.

Oh and it's cross-platform too--it's already been compiled on Linux and OSX successfully.